Field of the Invention
The present invention relates to a sheet processing apparatus that performs post processing on sheets having images formed thereon, and an image forming system including the sheet processing apparatus.
Description of the Related Art
Conventionally, there has been widely known a sheet processing apparatus, such as a copy machine or a printer, that is disposed downstream of an image forming apparatus, and performs post processing, such as stitching, on sheets output from the image forming apparatus. In recent years, sheet processing apparatuses have come to be equipped with more functions, and there has been proposed sheet processing apparatuses capable of performing not only conventional side stitching, but also saddle stitching. Further, the sheet processing apparatuses capable of performing saddle stitching include those which are further equipped with a bookbinding function for folding a stitched sheet bundle at a saddle-stitched portion to thereby form the sheet bundle into a booklet.
Further, there has been known a press processing technique in which a sheet bundle formed by a plurality of sheets stacked on a sheet stacking tray is conveyed while being folded, and a pressing roller is moved while pressing a fold line portion of the sheet bundle in a direction orthogonal to a conveying direction (thickness direction) to thereby improve the fold properties of the fold line portion. Further, U.S. Pat. No. 7,431,274 describes a technique in which in parallel with the press processing for pressing the fold line portion of the sheet bundle from opposite sides in a direction perpendicular to the sheet surface of the sheet bundle, there is performed flattening processing for flattening the fold line portion of the sheet bundle by pressing the fold line portion in a direction orthogonal to the thickness direction of the sheet bundle (direction opposite to the sheet conveying direction) using a shaping roller. A unit including the folding rollers (pressing rollers) and the shaping roller moves along the fold line portion, whereby it is possible to perform press processing and flattening processing in parallel, and make the fold line portion square-cornered, which further improves the fold properties.
An amount of pressure (pressing strength) applied to a fold line portion in flattening processing is determined according to a stop position, i.e. a processing position of a sheet bundle when the fold line portion of the sheet bundle is pressed. The amount of pressure can be set by a user as desired. However, when the set amount of pressure is large and also the sheet bundle has a large thickness, an operation load applied to the unit is sometimes larger than a driving force from a motor for moving the unit. In this case, the unit is incapable of moving along the fold line portion, which not only makes it impossible to properly perform the processing, but also brings about a possibility of erroneous detection of a failure of the unit in spite of the fact the unit is not in failure.